Slave network slave processing method and apparatus information collection method

ABSTRACT

A network system is constructed by connecting a safety PLC constituting a master unit and safety slaves  2  to each other through a safety network  3.  The safety slaves each have a device information storage unit  25  for storing individual information and status information of the safety devices connected. The individual information is acquired and stored in advance, while the stored contents of the status information are updated based on the result of monitoring the status of the safety devices in operation. These processes are executed by a MPU  23.  In response to a request from the safety PLC, the device information stored are transmitted, so that the safety PLC can collect and manage the device state as well as the normal state of the slave units.

TECHNICAL FIELD

The present invention relates to slave units and a network system aswell as a slave unit processing method and a device informationcollecting method.

BACKGROUND ART

In the programmable controller (hereinafter, referred to as “PLC”) usedfor the factory automation (hereinafter, referred to as “FA”), theon/off information is input from an input device such as a switch or asensor, and the logic operation is executed according to a sequenceprogram (also referred to as the user program) written in the ladderlanguage or the like. In accordance with the operation result obtained,the signal of on/off information is output to an output device such as arelay, a valve or an actuator thereby to perform the control operation.

The PLC is connected to the input device and the output device eitherdirectly or through a network. In the case where a network systemconnected by a network is constructed, the on/off information istransmitted or received through the particular network. In the process,the information is transmitted by the master-slave method in which thePLC normally acts a master unit and the device as a slave unit.

Recently, on the other hand, a fail-safe system has been introduced inthe control operation with PLC. Specifically, the network as well as thePLC and each device are configured of a built-in safety function. Thesafety function is defined as a function to confirm safety and outputdata. The safety system is such that in the case where an emergency stopswitch is depressed or a sensor such as a light curtain detects theintrusion of a person (a part of the human body) and the network systemfaces a dangerous situation, the fail-safe function works and the systemturns to the safety side, thereby stopping the operation. In otherwords, the safety function described above allows a signal to be outputonly in the case where safety is secured in storage and to start theoperation of a machine. In the case where no safety is assured,therefore, the machine stops.

The state of a given slave unit can be returned as a response by theslave unit that has received the request from PLC (master unit). As aresult, PLC can acquire the status of the slave units connected to anetwork.

In the conventional network system, though the informed on the slaveunits connected can be recognized, the information on the input deviceor the output device connected to a slave unit cannot be recognized fromPLC (master unit). The slave unit can be informed of the fact that amalfunction or a fault has occurred. In case of a malfunction,therefore, it is necessary to resort to the site of installation andspecify the cause of the malfunction. In the case where a given part isrequired to be changed, an order for the part is issued and, with thenewly acquired part, the installation site is visited again to changethe part. This maintenance is complicated, and makes it difficult totake the necessary action quickly. It is still more difficult to monitorthe life of the input and output devices connected to a slave unit fromPLC, and therefore, a malfunction or fault is often handled only afterit has actually occurred.

Even in the case where the installation site of a slave unit or a deviceconnected with the slave unit is visited upon occurrence of a fault orthe like, therefore, the status cannot be easily checked if the deviceis small or installed in the depth or behind other devices. As a result,the status of these input and output devices cannot be accuratelygrasped, and therefore the cause of a malfunction or fault may not beanalyzed sufficiently.

Further, in the case of a network system having security means, thesafety devices, though high in reliability and redundancy, are oftenshorter in life than the devices making up the conventional normalnetwork. Once a malfunction of these safety devices occurs, it leads tothe stoppage of the whole system, and therefore has a greatdisadvantageous effect on the whole system. Consequently, the functionof monitoring the devices connected to a slave unit is more crucial thanin the conventional system.

An object of this invention is to provide a slave unit and a networksystem as well as a slave unit processing method and a deviceinformation collecting method, in which the information on each deviceconnected to a slave unit can be collected through a network.

DISCLOSURE OF THE INVENTION

In order to achieve the aforementioned object, according to thisinvention, there is provided a slave unit connectable to a network,comprising information collecting means for collecting the informationon the devices connected, device information storage means for storingthe device information collected by the information collecting means,and means for outputting the information stored in the deviceinformation storage means through the network. The informationcollecting means corresponds to a MPU 23 in the embodiments.

Preferably, the device information include at least one of theindividual information for specifying the devices and the statusinformation for indicating the device status. The individual informationis the one for specifying the model, the maker name, the type, thespecification, etc. of a particular device. These device information areregistered in the slave unit in advance and output as required. Thestatus information, on the other hand, is the one for specifying thedevice status including the on/off state, the life information such asthe turn-on time and the number of times operated and the result ofself-diagnosis. This information can be acquired by monitoring thedevices while the system is in operation. In other words, the deviceinformation collecting means has the function of monitoring the devicestatus in real time and the function of acquiring the individualinformation prior to the start of the operation.

The network system according to this invention is constructed of acontroller and the slave units described above connected to each otherthrough the network. The device information output from the slave unitsare transmitted to the controller.

According to this invention, the information on the devices connected tothe slave units are stored and held in each slave unit, and theparticular information stored and held can be transmitted to otherdevices (such as a controller and a tool) through the network. Withoutvisiting the site where each slave unit and the devices connectedthereto are installed, therefore, the device information can beacquired. Once a malfunction is found in a given contact of a slaveunit, the name of the device having the malfunction can be determinedbased on the information of the devices connected to the particularcontact. Therefore, the maintenance job can be quickly carried out bypreparing a normal device of the same type as the defective device andbringing it to the installation site. The status information can clarifythe cause of an abnormal halt, while the notification of the servicelife information can prevent a malfunction which otherwise might occurupon expiry of the device life.

Further, the slave processing method according to this invention is themethod of processing each slave unit connected to a network, wherein thecollecting process for collecting the information on the devicesconnected to the particular slave, the process for storing in the deviceinformation storage means the device information obtained by executingthe collecting process and the process of outputting the informationstored in the device information storage means through the network areexecuted.

Also, the device information collecting method according to thisinvention is employed for a network system constructed by connecting acontroller and slave units to each other through a network, wherein theslave unit executes the collecting process for collecting theinformation on the devices connected to each slave unit, the process forstoring in the device information storage means the device informationobtained by executing the collecting process and the process ofoutputting the information stored in the device information storagemeans through the network. The controller acquires and stores the deviceinformation output from the slave unit.

As another solving means, there is provided a device informationcollecting method for a network system constructed by connecting acontroller and slave units to each other through a network. This networksystem is connected with a monitoring device, and the controller and theslave units are controlled by transmitting and receiving the controlinformation such as the I/O information. Each slave unit executes thecollecting process for collecting the information on the devicesconnected to the particular slave unit, the process for storing thedevice information obtained by executing the collecting process in thedevice information storage means and the process of outputting theinformation stored in the device information storage means through thenetwork. The monitoring device acquires and stores the deviceinformation output from the slave unit.

The monitoring device is connected to the controller, and can indirectlyacquire the device information through the controller. The deviceinformation can also be collected directly by connecting the monitoringdevice to the network to monitor the frames transmitted on the networkand causing the monitoring device to receive the device information senttoward other nodes such as the controller. The monitoring device, ifconnected to the network, can of course acquire the device informationdirectly by causing a slave unit to transmit the particular deviceinformation to the monitoring device. According to the embodiments, themonitoring device corresponds to a monitoring tool 5, a configuration.tool 31, a monitoring device 33, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a network system according to an embodimentof this invention.

FIG. 2 is a diagram showing a slave unit according to an embodiment ofthe invention.

FIG. 3 is a diagram showing the data structure of a device informationstorage unit.

FIG. 4 is a flowchart showing the functions of a MPU.

FIG. 5 is a flowchart showing the functions of a MPU.

FIG. 6 is a diagram showing an example of a system configuration forinitial registration in a slave unit.

FIG. 7 is a diagram showing an example of the internal data structure ofa data base.

FIG. 8 is a flowchart showing a specific processing function for theinitial registration process.

FIG. 9 is a diagram showing an example of the data structure stored in adevice information storage unit 25.

FIG. 10 is a diagram showing a network system according to anotherembodiment of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

This invention will be described in more detail with reference to theaccompanying drawings.

FIG. 1 shows an example of a safety network system to which theinvention is applicable. As shown in the figure, a safety PLC 1 (masterunit) and a plurality of safety slave units 2 are connected to eachother through a safety network 3. The safety PLC 1 and the safety slaveunits 2 transmit and receive. information to and from each other by themaster-slave method. Further, each safety slave unit 2 is connected withvarious safety devices 4 such as input devices and output devices inaddition to a safety door switch, a safety limit switch and an emergencystop switch. It is noted that the safety PLC 1 is configured byconnecting a plurality of units including, for example, a CPU unit, amaster unit (communication unit) and an I/O unit. In this case, themaster unit is connected to the safety network 3.

Further, a monitoring tool (such as a personal computer) 5 isconnectable to the CPU unit and the master unit of the safety PLC 1.This monitoring tool 5 collects and manages the information on thesafety slave units 2 and the safety devices 4 connected thereto, throughthe safety PLC 1 as described later.

The various devices making up this safety network system all have abuilt-in safety (fail-safe) function. This safety function isconventionally well known and therefore is not described in detail, butonly the transmission and reception of information which constitute themain parts of the invention.

The safety slave units 2 each have a plurality of terminal bases 2 awhich are connected with various safety devices 4. The safety slave unit2 has the internal structure as shown in FIG. 2. As shown in the samefigure, the safety slave unit 2 includes a communication unit 21connected to the safety network 3 for transmitting and receiving data toand from the safety PLC 1 (master unit), an input/output circuit 22 fortransmitting and receiving data to and from the safety devices 4connected to the safety slave unit 2, and a MPU 23 for executing apredetermined process by reading the program stored in a system ROM 24.The MPU 23, in compliance with the request addressed to itself throughthe communication unit 21, executes the process of returning theinformation (safety information, etc.) acquired from the safety devices4 through the input/output circuit 22 to the safety PLC 1 (master unit)through the communication unit 21 and the safety network 3.

Further, the MPU 23 has the self-diagnosis function and the function ofmonitoring the operating conditions (the turn-on time, the number oftimes turned on/off, etc.) of the safety devices 4, and also executesthe process of storing the device information such as the diagnosisresult and the operating conditions obtained by the operation of eachfunction in a device information storage unit 25. The unsafetyinformation (device information) stored in this device informationstorage unit 25 are also returned in compliance with the request of thePLC 1 thereby to transmit the device information to the safety PLC 1. Itis noted that this device information storage unit 25 can be configuredof, for example, an EEPROM.

Furthermore, the information stored in the device information storageunit 25 is not limited to the operating conditions, etc. of the safetydevices 4 described above but also include individual information suchas the type, model name and the maker name of the safety devices 4connected.

The device information storage unit 25 has a data structure as shown,for example, in FIG. 3. Inputs 1, 2, 3, . . . designate the numbers ofcontacts (terminal bases 2 a). Of the shown items to be registered, thetype, the maker name, the model and the life setting are registered inadvance. Specifically, using a tool connected to the safety PLC 1 or thesafety network 3, for example, the required information is sent to thesafety slave units 2 through the safety network 3, or from the tooldirectly connected to the safety slave unit 2. The MPU 23 of the safetyslave unit 2 thus acquires the same information through thecommunication unit 21, and registers the particular information asrelated to the contact number in the device information storage unit 25.The life setting includes, for example, the turn-on time, the number oftimes operated or a value obtained by a predetermined formula from theturn-on time or the number of times operated.

The state, the number of times operated, the turn-on time, theself-diagnosis information and the notice flag are collected andrecorded by the MPU 23 while the actual system is in operation. Thestate is the information for discriminating whether the safety devices 4are operating (in on state) or not (off information), the number oftimes operated is the information indicating the number of times thecontacts of the safety devices 4 are turned on/off, and the turn-on timeis the accumulated time when the safety devices 4 are turned on.Further, the self-diagnosis information is the result of diagnosisobtained by carrying out the self-diagnosis function of the safety slaveunits 2 and diagnosing the safety devices connected, or in the casewhere a given safety device 4 has the self-diagnosis function, theresult of diagnosis sent from the particular safety device 4. Further,the result of life stored is indicative of whether the life has expiredor not (normal). The expiry of life indicates either a case where lifehas actually expired or a case where the expiry of life is near.Furthermore, the safety slave unit 2 has a display unit 26 which candisplay the device information stored in the device information storageunit 25.

The specific functions of the MPU 23 for executing the process describedabove are shown in the flowcharts of FIGS. 4 and 5. Specifically, thefirst step is the initial registration (ST1). In this initialregistration, the individual information on the safety devices suppliedfrom the tool are acquired and registered in the device informationstorage unit 25 as related to the contact number.

Once the actual control operation is started, the normal safety functionprocess (ST2), the unsafety information confirmation/collecting process(ST3) and the information updating process (ST4) are executed. It isthen determined whether there is a request from a higher level of thenetwork, that is, whether there is a request from the safety PLC 1(ST5). Specifically, as well known, the master-slave method beinginvolved, the safety PLC 1 which is a master unit sequentially issues arequest to the safety slave units 2 connected to the safety network 3 inone communication cycle, and receives the response from the safety slaveunits to which the request is issued, thereby transmitting and receivingthe data. In the case where there is a request addressed to the very MPU23 (Yes in step 5), therefore, the information meeting the request istransmitted to the safety PLC (ST6). After this transmission, or in theabsence of a request to the MPU 23 itself, the process is returned tostep 2. After that, the aforementioned process is repeatedly executed.

A more specific sequence of the safety function process (ST2), theunsafety information confirmation/collecting process (ST3) and theinformation updating process (ST4) is shown in FIG. 5. As shown in thesame figure, the MPU 23 makes self-diagnosis and monitors the safetyinput (ST10). Specifically, the self-diagnosis is the inspectionconducted to see whether a malfunction has occurred in the safetydevices 4 connected, and is a well-known process. The monitoring of asafety input, on the other hand, is to monitor the input from the safetydevices 4 connected. In the case of a malfunction or an input, aparticular safety device 4 with which the information is associated isalso specified.

Then, it is determined whether as the result of self-diagnosis or thesafety input monitoring in step 10, a malfunction is detected or thesafety input is off (not safe or dangerous) (ST11). In the case where amalfunction or the like is detected, the safety halt process is executed(ST12). In other words, the fail-safe function works to halt theoperation. Also, the particular malfunctional state is stored in thecolumn of the self-diagnosis result or the on/off information of thecorresponding contact number in the device information storage unit 25.The diagnosis result, etc. correspond to the request for the safetyinformation from the safety PLC 1 and are transmitted as a safetyresponse. The self-diagnosis information (the safety device that hasdeveloped a malfunction and the contents of the malfunction) are outputto and displayed on the display unit 26. The process of step 10 to step13 represents the safety function process for step 2.

In the case where the branching decision in step 11 is “No”, that is,the safe state prevails, the turn-on time is updated (ST14).Specifically, the time from the preceding update process to the presenttime, for example, is measured by the timer (temporarily stopped withoutcounting during a halt (not turned on) of the safety device 4), and thesum of the turn-on time for the preceding update process and the turn-ontime from the update process to the present time is set as a new turn-ontime, and this new turn-on time is stored in the device informationstorage unit 25.

It is determined whether the input state of a safety device 4 has turnedon (present session) from off state (preceding session) (ST15).Specifically, in the case where the input state of the safety device 4is off in the preceding session, the data in the on/off informationcolumn of the device information storage unit 25 is turned off throughthe process of step 12. Therefore, in the case where the on/offinformation of the corresponding contact number is off, the branchingdecision is “Yes”. Then, the process proceeds to step 16, and theoperation count is incremented by one (ST16). This operation count isregistered in the “number of times operated” column of the deviceinformation storage unit 25, while at the same time turning on the“state” column of the device information storage unit 25. As a result,when the operation count is incremented by one, so that the “state”column turns on. In the case where the branching decision of step 15 isexecuted in the next cycle (the safety input cannot be turned offmidway), therefore, “No” is the answer and the operation count is notcarried out. Also, since the ON state is registered in advance, there isno need of writing “ON” again.

After the above-mentioned operation count or in the absence of inputstate change (No in step 15), the process proceeds to step 17 todetermine whether the number of times operated or the turn-on time hasreached a value requiring a life warning or not. The value requiring alife warning (threshold value) is set for each safety device in the“life setting” column of the device information storage unit 25. Such avalue is compared with the current number of times operated and theturn-on time, and it is determined whether the life setting has reachedthe value requiring a life warning or not.

In the case where the value requiring a life warning is not yet reached,the process returns to step 10. In the case where it is determined thata life tendency is required, on the other hand, the process proceeds tostep 18, and the “result of life” column of the device informationstorage unit 25 for the safety device whose life has expired of thedevice information storage unit 25 is updated from “normal” to“abnormal”. This result is also sent in compliance with the request fromthe safety PLC. The processes of step 14 to step 18 described aboveconstitute the unsafety information conforming/collecting process (ST3)and the information updating process (ST4).

Further, a more specific sequence of the initial registration process instep 1 of FIG. 4 is as described below. Specifically, as shown in FIG.6, the initial registration is made in the slave units 2 using theconfiguration tool 31 connected through the safety network 3. In otherwords, the configuration tool 31 has a device data base 32 a and adevice-related information data base 32 b. These data bases 32 a and 32b may be configured as a physically single memory or as separatememories. These data bases can of course be configured of either theinternal and external storage units of the configuration tool 31 orindependent storage units installed on the safety network 3.

The device data base 32 a has stored therein the information fordefining the safety slave units 2 and other network devices and theinput/output devices connected to the network, and all the informationsupported by each of these devices. The items of these informationinclude the device name, the vendor name, the model name, the settableindividual parameter, the connectable input/output device informationand the life information.

The device-related information data base 32 b, on the other hand, hasstored therein the information for relating the network devices such asthe slave units to the input/output devices connected thereto and thesetting information. The items of these information include the I/Oassignment information and the individual setting information. The datastructure of these information is such that, as shown in FIG. 7, theinformation on the slave units (node #00) providing the network devicesand the information on the input/output devices connected to the slaveunits (node #00) are sequentially stored.

The user, when making initial registration in the safety slave unitsconnected to the safety network 3 using the configuration tool 6,collects the information on the slave units to be registered, from thedata bases 32 a and 32 b and transmits the collected information to thecorresponding safety slave units through the safety network 3 asconfiguration information. Then, each safety slave unit 2, as shown inthe flowchart of FIG. 8, waits for the reception of the configurationinformation (ST21). Upon reception of the configuration information,each safety slave unit 2 registers the received configurationinformation in the device information storage unit 25 thereof (ST22).The completion of this registration terminates the various registrations(ST23). As the result of completion of this registration, the deviceinformation storage unit 25 is held with the data structure as shown inFIG. 9, for example. Specifically, the information on the slave unititself (registered in M and subsequent addresses) and the information onthe input/output devices connected thereto (registered in N andsubsequent addresses) are held. The process of steps 21 to 23corresponds to the initial registration process of step 1 shown in FIG.4.

The other desirable information for each I/O terminal of the salve unitsthan those on the slave units registered in this initial registrationprocess and those on the input/output devices connected to theparticular slave units, shown in FIG. 3, include the following variousones. First, for example, the information by type includes theinformation indicating the terminal ID. This includes the terminal No.,the terminal attributes (input, output, AC, DC, semiconductor, contact,etc.), the screw size, the signal name, etc. Also, the informationindicating the ID of the devices connected to each I/O terminal is alsoincluded. Specific ones are the device name, the maker name, the model,the serial No., the address and the sub-address. Further, theinformation registered as comments include, for example, the diameter ofthe wiring cable, the color of the wiring cable, the device controlarea, the device to be controlled, the magnitude of the possible dangerof the device to be controlled. Also, the information set as theoperating conditions include, for example, the threshold value (turn-ontime, number of times turned on/off, leakage current, light amount,etc.). Further, the numerical information that can be counted includethe number of times a device is turned on/off, the turn-on time of thedevice, the frequency at which the device is used. Also, the numericalinformation subjected to secular variation include, for example, theleakage current of a device, the light amount of the device, theresponse time of a machine, and the restoration voltage of the device.The present status information include “on or off”, the analog value,the result of self-diagnosis (short-circuiting, disconnection,malfunction, etc.).

According to this embodiment, the safety PLC 1 gives a request forunsafety information (device information 4), and thus can collect thedevice information stored in the device information storage unit 25 ofthe safety slave unit 2. Thus, the collected information can be outputto and displayed on the monitoring tool 5. Even at a remote place,therefore, the individual information (model, specification) and theconditions of the safety devices connected can be monitored and thecause of the malfunction can be clarified. Specifically, a defectivesafety device can be determined without visiting the installation siteof the device. By visiting the installation site with a spare part forreplacing the defective part, therefore, the restoration process such asthe change of the safety device that has developed the malfunction canbe quickly executed. Also, the notification of the life information canprevent the malfunction related to the device life.

Furthermore, the notification of the network information (the number ofretrials made in case of a communication error, input/output responsetime, etc.) can clarify the points for improving the network environmentand optimize the response time (safe down time for a safety protectionsystem). Specifically, a safety device that frequently develops amalfunction or a fault is considered to harbor some problem, and in sucha case, the particular device or the whole system can be changed.

Also, especially in the safety slave units, by making the most of theself-diagnosis function that has not been used conventionally asmonitoring information, the causes of a device halt can be desirablygrasped and notified by being classified into a halt by the safety inputoff operation, a halt due to a malfunction of the input/output circuitand a halt due to a malfunction of the communication between a masterunit and a slave unit.

In the above-mentioned embodiments, an example of application as asafety network system is described. Nevertheless, the invention is ofcourse applicable also to an ordinary network system lacking such safetyfunctions.

Also, either one or a plurality of devices may be connected to one slaveunit. The effects of this invention are exhibited more conspicuously,however, in the case where a plurality of devices are connected and theinformation on the devices connected to each contact can bediscriminately known as in the embodiments described above.Specifically, even in the case where it can be detected whether a slaveunit has developed a malfunction or not and which contact of the slaveunit has developed a malfunction, it has thus far been impossible todetermine which device is in what state without confirmation at theinstallation site as long as the device information is lacking. In thisinvention, however, the state of each of a plurality of devices, if any,can be acquired through a network.

Further, the foregoing description concerns the master-slave method inwhich the slave units described in the embodiments above are shown, asan example, to transmit and receive the I/O information to and from themaster unit, and the system is controlled by transmitting and receivingthe I/O information to and from the controller (PLC) through the masterunit. In this method, the master unit and the slave units are so relatedto each other that the desired slave unit gives a response to therequest of the master unit. The slave units referred to in thisinvention, however, are not limited to those for communication betweenthe master unit and the slave units. In other words, although the term“slave unit” is used, an arbitrary communication method can be employed.In this respect, strictly speaking, the slave unit according to theinvention is considered to contain a different concept from the entitygenerally defined as a slave. Specifically, what is called the slaveunit in this invention can use any communication protocol for actualtransmission and receiving operation as long as it has the function oftransmitting and receiving the I/O information required for controloperation to and from the controller. Especially, the destinations ofinformation to be transmitted in this invention other than the I/Oinformation are not limited to the master unit or the controller, but asshown in FIG. 10, include a configurator (configuration tool) 31, amonitoring device 33 and other slave units and various nodes connectedto the network. The communication method can also be appropriatelyselected in accordance with the other party of transmission. Also, thetrigger for transmission is of course not limited to the one executed inresponse to an external request (external trigger), but transmission maybe based on the internal trigger (internal timer or an event generatedwhen predetermined conditions are met).

The “internal trigger” is based on the result of executing apredetermined process by a slave unit itself and generated within theslave unit. An example of the internal trigger is described below.Specifically, in the case where it is determined that the statusinformation of the input/output devices acquired by a slave unit hasreached a threshold value or whether it has exceeded a threshold valueor not, the result of determination is generated. This result ofdetermination is used as a trigger signal in some cases. Also, in thecase where the initialization process is executed by switching on powerfor a slave unit, the process of outputting the information stored in anonvolatile memory to a line or the process of generating a trigger maybe included in the initialization process. Further, in some cases, aclock is held in a slave unit, and by use of this clock, a triggersignal is generated upon the lapse of each predetermined time orperiodically or at a predetermined time point. In other cases, based onthe state of the communication traffic with the master, a trigger signalis generated in the case where the communication process has a margin ora fault such as an abnormal voltage.

The “external trigger”, on the other hand, is based on the commandreceived by a slave unit through a network, and generated outside theslave unit. An information request command from the master unit to theslave unit, an information request command from the monitoring unit tothe slave unit, the information request command from the configurator,and a command transmitted by a tool and sent through the PLC or themaster unit, are some examples of the external trigger.

Industrial Applicability

According to this invention, the following effects can be exhibited.Specifically, as described above, in this invention, the deviceinformation storage means is arranged in a slave unit to store and holdthe information on the devices connected to the slave unit. Therefore,the information on each device connected to the slave unit which are sostored and held can be collected by the controller or the tool throughthe network.

1. A slave unit connectable to a network, comprising: informationcollecting means for collecting the information on devices connected;device information storage means for storing the information on saiddevices collected by said information collecting means; and means foroutputting the information stored in said device information storagemeans through the network.
 2. A slave unit according to claim 1,characterized in that said device information include at least one ofindividual information for specifying the devices and status informationindicating the status of the devices.
 3. A network system constructed byconnecting a controller and the slave unit according to claim 1 to eachother through a network, characterized in that said device informationoutput from said slave unit is transmitted to said controller.
 4. Amethod of processing a slave unit connected to a network to execute: thecollecting process for collecting the information on the devicesconnected to the particular slave unit; the process of storing in thedevice information storage means said device information obtained byexecuting said collecting process; and the process of outputting throughsaid network the information stored in said device information storagemeans.
 5. A device information collecting method for a network systemconstructed by connecting a controller and a slave unit through anetwork, characterized in that said slave unit executes: the collectingprocess for collecting the information on the devices connected to theparticular slave unit, the process of storing in the device informationstorage means said device information obtained by executing saidcollecting process, and the process of outputting through said networkthe information stored in said device information storage means, andsaid controller acquires and stores said device information output fromsaid slave unit.
 6. A device information collecting method for a networksystem constructed by connecting a controller and a slave unit through anetwork, characterized in that said network system is connected with amonitoring device, said controller and said slave unit perform thecontrol operation by transmitting and receiving the control informationsuch as the I/O information, said slave unit executes the collectingprocess for collecting the information on the devices connected to theparticular slave unit, the process of storing in the device informationstorage means said device information obtained by executing saidcollecting process, and the process of outputting through said networkthe information stored in said device information storage means, andsaid monitoring device acquires and stores said device informationoutput from said slave unit.